1. Field of the Invention
The present invention relates to a control apparatus for a permanent-magnet synchronous motor, and more particularly to a synchronous motor control apparatus that performs current vector control of a permanent-magnet synchronous motor based on q-phase and d-phase current commands.
2. Description of the Related Art
Current vector control is known in the art as a method for controlling a three-phase permanent-magnet synchronous motor by using q-phase and d-phase current commands. FIG. 7 is a schematic diagram for explaining how the q-phase and d-phase current commands used for vector current control of a synchronous motor are related to three-phase current commands. In a control apparatus for controlling the three-phase permanent-magnet synchronous motor 3, a current vector control block 51 takes as inputs the q-phase current command iq and d-phase current command id created based on a torque command to the synchronous motor 3, the motor speed fed back from the synchronous motor 3, etc., and creates a q-phase current command iq* and a d-phase current command id* for achieving desired current vector control. A DQ/three-phase conversion block 52 performs a two-phase to three-phase conversion to convert the q-phase current command iq* and d-phase current command id* to current commands iu*, iv*, and iw* for the u-phase, v-phase, and w-phase, respectively, of the synchronous motor 3. Usually, the synchronous motor 3 is driven by an inverter apparatus (not shown), and the three-phase current commands iu*, iv*, and iw* output from the DQ/three-phase conversion block 52 are used as switching commands for controlling the switching operations of semiconductor switching devices provided in the inverter apparatus for the respective phases. The inverter apparatus supplies drive power to the respective phase windings of the synchronous motor 3 by performing the switching operations based on the three-phase current commands iu*, iv*, and iw*.
FIG. 8 is a vector diagram for explaining voltages generated in a permanent-magnet synchronous motor. More specifically, on a d-q current coordinate plane, an induced voltage ωΨa, which depends on the flux linkage Ψa of the permanent magnet and the electrical angular speed ω of the synchronous motor (hereinafter simply referred to as the “motor speed”) when the permanent-magnet synchronous motor is rotated at high speed, and a voltage ωLqiq, which depends on the motor speed ω and the q-axis inductance Lq and q-phase current iq of the synchronous motor, are generated. If their sum voltage V exceeds the voltage limit value Vom preset in the control apparatus, as shown in FIG. 8, a drive voltage deficient situation occurs, resulting in an inability to rotate the synchronous motor.
FIG. 9 is a vector diagram for explaining a servo motor current control method disclosed in Japanese Unexamined Patent Publication No. H09-084400. According to the method disclosed in Japanese Unexamined Patent Publication No. H09-084400, which achieves high-speed rotation of the synchronous motor by solving the above motor drive voltage deficiency problem, the d-phase current id is caused to flow, for example, in the negative direction of the d-axis to generate a voltage ωLdid that depends on the motor speed ω, d-axis inductance Ld, and d-phase current id, thereby reducing the motor drive voltage so that the voltage limit value Vom preset in the control apparatus will not be exceeded.
It is also well known that motor drive voltage deficiency can be addressed by “flux weakening control”, as described in “Design and Control of Interior Permanent Magnet Synchronous Motor” by Yoji Takeda, Nobuyuki Matsui, Shigeo Morimoto, and Yukio Honda and published by Ohmsha, first edition, fourth printing, pp. 17-27 and 38-46, 2007. According to the document “Design and Control of Interior Permanent Magnet Synchronous Motor”, when the motor speed of the synchronous motor is denoted by ω, armature flux linkage by Ψa, q-axis and d-axis inductances by Lq and Ld, respectively, the voltage limit value preset in the control apparatus by Vom, the q-phase current by iq, the d-phase current by id, and the induced voltage by Vo, and when the induced voltage Vo is held at the voltage limit value Vom, the relation defined by the following equation (1) is obtained.
                                                        (                                                                    L                    d                                    ⁢                                      i                    d                                                  +                                  Ψ                  a                                            )                        2                    +                                    (                                                L                  q                                ⁢                                  i                  q                                            )                        2                          =                              (                                          V                om                            ω                        )                    2                                    (        1        )            
In the flux weakening control, when the motor speed ω of the synchronous motor and the q-phase current command iq are given, the d-phase current command id is expressed as shown by the following equation (2).
                              i          d                =                                            -                              Ψ                a                                      +                                                                                (                                                                  V                        om                                            ω                                        )                                    2                                -                                                      (                                                                  L                        q                                            ⁢                                              i                        q                                                              )                                    2                                                                          L            d                                              (        2        )            
The flux control given by the above equation (2) can consider the voltage limit but cannot consider the current limit. If both the voltage limit and the current limit are to be considered, one possible solution is to employ a control method, such as described in the document “Design and Control of Interior Permanent Magnet Synchronous Motor”, that switches the control mode according to the motor speed ω of the synchronous motor. FIG. 10 is a schematic diagram for explaining the method of controlling the synchronous motor having a plurality of control modes described in the document “Design and Control of Interior Permanent Magnet Synchronous Motor”. According to the description given in the document “Design and Control of Interior Permanent Magnet Synchronous Motor”, if maximum output control that obtains maximum output by considering the voltage and current limits is to be achieved, the current vector must be controlled appropriately according to the motor speed ω of the synchronous motor. More specifically, a speed judging unit 53 that judges the motor speed ω of the synchronous motor is provided within the synchronous motor control apparatus, and when the motor speed ω is not greater than the “base speed ωbase that reaches the voltage limit value”, the q-phase current command iq and the d-phase current command id are created based on a control mode I that maximizes torque by considering only the current limit; on the other hand, when the motor speed ω is greater than the base speed ωbase but not greater than a predetermined high speed ωd, the current commands are created based on a control mode II that maximizes torque by considering both the current limit and the voltage limit, and when the motor speed ω is greater than the predetermined high speed ωd, the current commands are created based on a control mode III that maximizes torque by considering only the voltage limit.
As a method similar to the above flux weakening control, a control method that varies the d-phase current according to the torque command is disclosed, for example, in Japanese Unexamined Patent Publication No. 2003-052199. According to this control method, a maximum load d-phase current command value that controls the motor terminal voltage at maximum load to within the maximum output voltage of the motor control apparatus and a minimum load d-phase current command value that controls the motor terminal voltage at no load to within the maximum output voltage of the motor control apparatus are obtained according to the number of revolutions of the motor, and a value interpolated from these two d-phase current command values according to the magnitude of the torque command is taken as the d-phase current command.
On the other hand, Japanese Unexamined Patent Publication No. 2006-020397, for example, discloses a method for controlling a permanent-magnet synchronous motor, in which the AC supply voltage to be input to a power amplifier or the DC link voltage obtained by rectifying the input voltage is measured, and the reactive current (d-axis current) or the amount by which to advance the current control phase is varied according to the supply voltage, thereby directly controlling the reactive current or phase in accordance with the variation of the input supply voltage.
Further, there are proposed several methods that impose a limit on the d-axis current command in accordance with motor speed by using a prescribed mathematical equation, for example, as disclosed in Japanese Unexamined Patent Publication No. 2008-086138.
There is also proposed a control method, such as disclosed in WO 2008/038338, that achieves the maximum torque/current control described in the document “Design and Control of Interior Permanent Magnet Synchronous Motor” through calculations using approximate equations.
According to the control method that switches the control mode according to the motor speed ω of the synchronous motor, as described in the document “Design and Control of Interior Permanent Magnet Synchronous Motor”, the base speed ωbase and the speed ωd by reference to which the mode is switched between control mode II and control mode III are predicted and set in advance, and the q-phase current command and the d-phase current command are computed independently by switching the control mode. This gives rise to the problem that if the base speed ωbase and the speed ωd were not correctly predicted, current flow would be interrupted when switching the control mode, causing the synchronous motor to vibrate.
Furthermore, the mathematical equation such as equation (1), which involves operations such as the calculation of the square root and divisions, makes the program complex, and the computation speed is slow. It is therefore difficult to operate the synchronous motor control apparatus with a short control period, and hence it is difficult to enhance the controllability of the permanent-magnet synchronous motor.